Detection of H2S, HF and H2 pollutant gases on the surface of penta-PdAs2 monolayer using DFT approach

In this research, the adsorption of targeted noxious gases like H2S, HF and H2 on penta-PdAs2 monolayer are deeply studied by means of the density functional theory (DFT). After the capturing of three kind of pollutant gases (H2S, HF and H2), it is observed that, the electronic properties are slightly affected from the pristine one. In all cases, the physisorption interaction found with adsorption energy of − 0.49, − 0.39 and − 0.16 eV for H2S, HF and H2 gases, respectively. Which is exposed that H2S gas strongly absorbed on penta-PdAs2 nanosheet. In case of HF (H2) gas adsorbed systems, the obtained charge transfer is + 0.111 e (+ 0.037 e), revealed that the electrons are going to PdAs2 nanosheet from the HF (H2) molecules. Further, under the non-equilibrium Green’s function (NEGF) theory, the IV response and sensitivity of absorbed H2S, HF and H2 have been discussed. The results demonstrate that the H2S molecules on PdAs2 has suitable adsorption strength and explicit charge transfer compared with other targeted molecules. Hence, our novel findings of H2S, HF and H2 targeted gas sensing on penta-PdAs2 nanosheet might provide reference-line to design modern gas sensor device at the nano-scale.

H 2 S, HF and H 2 are toxic and colorless gases, and they are common industrial and environmental pollutants, and also became extensive range sources of health effects [1][2][3] . Additionally, due to rapid industrialization, universally use of chemicals, transportation of automobiles has led to increasing global environmental pollution and diminution of the ozone layer, which has expected a great hazard and major threat to the human health [4][5][6] . However, the arena of gas sensing has always concentrated on the research in control of environmental pollution, monitoring of industrial activities, health diagnostics and noxious gas sensing's sake of public safety [7][8][9] . In recent years, numerous nanomaterials have been scrutinized for gas sensing applications. In spotlight, the two-dimensional (2D) nanomaterials have castoff to sense a low concentration of gas molecules in sensing applications because of their compensations like, high stability, novel structures, large surface area/volume ratio, excellent adsorption abilities and low cost [10][11][12][13][14][15][16][17][18][19] . For example, graphene 20 , Phosphorene 21 , carbon nitride compound 22 , B 4 C 3 monolayer 23 , InN monolayer 24 and transition-metal dichalcogenides (TMDs) like MoSe 2 , PtSe 2 , SnS 2 and HfS 2 , etc. [25][26][27][28][29][30] , have become the burgeoning materials as toxic gases sensor.
Moreover, the key foundations of (i) H 2 S gas are from volcanic gas, fuel, sewage plants, sulfur deposits and ammonia synthesis from hydrocarbon feedstock [31][32][33][34][35][36][37] . Although, very recently, a study shows B 4 C 3 nanosheet, as excellent gas sensor for CO, NO, NH 3 , SO 2 and H 2 S 23 and Gao et al. 38 , testified the strong adsorption capacity of H 2 S and CH 4 with the modified 2D graphene sheet. Then, (ii) HF gas is also eminent toxic gas it is mainly originate from industrial processes at high temperature and the combustion of products containing fluoride, which is harmfully affect to not even only human but also flora and fauna. The high exposure of HF gas causes, muscle spasms and result may even in fatality at extreme cases 2 .
Bhattacharya et al. 38 , also studied the sensing of HF, HCN and H 2 S with nitrogenated holey carbon (C 2 N-h2D) monolayer. Next, (iii) H 2 a tasteless, odourless and flammable gas. Even, by lacking O 2 concentration in enclosed area, only 4% concentration of H 2 gas could be reasons to burnt and suffocation 39 . Li et al. 40 also explored Rudoped PtSe 2 monolayer as good sensor of H 2 gas and Pandey et al. 41 also suggested that MoS/WS monolayer is potential substrates for gas sensing of H 2 , NO and CO gases. www.nature.com/scientificreports/ Apart from aforementioned two-dimensional (2D) materials gas sensor, the newly discovered pentagonal structures had been also showed to design gas sensor due to its superior quality such as high surface carrier mobility, large surface area, more adsorption sites and great optical properties over the traditional unit hexagon structure 42,43 . Additionally, in past, several 2D pentagon unit structure has also been reported for the promising candidates for innovative applications in the gas sensor. Like, Wei et al. 44 predicted that penta-BCN monolayer is good sensor of CO, H 2 S, NH 3 and NO gas molecules and stated that the targeted gases have moderate adsorption energies in the range of − 0.797 eV to − 1.186 eV and proved it as potential applicant for said gas molecules sensors applications. Lakhani et al. 45 shows the dissociation of air pollutants on the uniform surface of pentagonal BeP 2 monolayer using first principles study. Xia et al. 46 also explored that penta-PdSe 2 monolayer has meaningful and promising material to be applied in FET type gas sensors as detection of NO 2 gas. Then, Tang et al. 47 revealed the new sensor material P-SiC 2 monolayer and concluded that after the contact with NO 2 gas the electronic resistance of P-SiC 2 monolayer is decreased significantly, which indicating the ultrahigh sensitivity towards NO 2 sensing. Hereafter, inspired by these Penta two-dimensional (2D) gas sensing research work owing to their robust adsorption capabilities and sensitivity, herein we are first time examined H 2 S, HF and H 2 gas sensing capabilities on the newly discovered novel penta-PdAs 2 monolayer, because modelling new gas sensing resources have paramount bid to probe the pollutant molecules in the current era. Viz, H 2 S, HF and H 2 gases. And finding an effective material to detecting and adsorbing these toxic (H 2 S and HF) and highly flammable (H 2 ) gases are great worth to protect our living environment for modern civilization. Even though, the outstanding electronic properties, transport, and optical properties of pristine penta-PdAs 2 monolayer is previously done by our research group 48 .
In this work, the capacity of monolayer penta-PdAs 2 as gas sensor (of H 2 S, HF and H 2 gases) capture is discovered by first-principles calculations. As density functional theory calculation is a widespread and feasible approach for simulation of gas adsorption on the novel nano-surfaces. Henceforth, the adsorption process of H 2 S, HF and H 2 gases on penta-PdAs 2 monolayer was studied and compared by evaluating differences in adsorption energy (E ad ), appearance change (e), charge transfer (Q), work function (Φ), charge density difference (CDD), electronic band structure, density of states/eV (DOS) and partial density of states/eV (PDOS) in this research note. These observations deliver quest of sensor materials with high sensitivity and selectivity towards pollutant gases such like H 2 S, HF and H 2 .

Simulation details
Density Functional theory (DFT) calculations were executed using SIESTA code 49 to examine the optimization, electronic and adsorption (of targeted gases) properties of penta-PdAs 2 nanosheet. To treat the exchange-correlation (XC) effect, the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) 50 functional was employed. We incorporated the DRSLL dispersion correction method to treat the long range vdWs forces 51 . The Kohn-Sham one electron states were expanded in a plane-wave basis set. The plane wave kinetic energy cut-off is considered as 450 Ry. The doubled zeta plus (DZP) basis set was used with an energy of 0.01 Ry to expand the Kohn-Sham orbital. The Brillouin zone sampled using a 5 × 5 × 1 Monkhorst-Pack k-point mesh during structural optimization 52 . The k-point mesh was then increased to 15 × 15 × 1 to attain more accurate results for electronic structure simulations. The optimized structures of 3 × 3 × 1 supercell penta-PdAs 2 monolayer is shown in Fig. 1a. As aforementioned, potential properties of penta-PdAs 2 monolayer such as structural, electronic, transport and optical properties have already reported by our group 48 and proven that it is dynamically stable via the positive phonon frequencies of penta-PdAs 2 . Hence, using same substrate of penta-PdAs 2 , herein we reported the sensing potential in the penta-PdAs 2 monolayer for H 2 S, HF and H 2 gases. All the optimized structures were relaxed without given any geometric constraint (GC) on the sheet. The atomic force is specified as 0.01 eV/Å to relaxing the systems. A vacuum of 15 Å have been considered along the perpendicular direction of the penta-PdAs 2 surface to avoid the interaction of periodic boundary conditions as employed in XY-plane. The adsorption energy is determined by the following equation 53 : where, E nanosheet+Gas represents the total energy of the penta-PdAs 2 nanosheet with the gas molecules absorbed on, while E nanosheet and E Gas are the total energy of penta-PdAs 2 sheet without the gas molecules and the isolated gas molecules, respectively. From the adsorption energy definition, a negative E ads specifies an exothermic process and thermodynamically favourable for molecular gas sensing. All the initial distances between sheet and gas molecules are set to 2.0 Å. The Hirshfeld atomic population analysis method was used to estimate the charge transfer (Q) as defined in SIESTA and negative value of Q means that electron transfer sheet to gas molecules. The charge density difference (CDD) Δρ between the adsorbed and isolated gases is calculated by: where, ρ nanosheet+Gas , ρ nanosheet and ρ Gas are represent the charge densities of penta-PdAs 2 monolayer with, without the adsorption of gas and charge density of gas molecules, respectively. This CDD has been imagined with utility of VESTA software 54 . The recovery time (τ) is the crucial parameter for gas sensor, which can be measured experimentally ranging from second to several minutes. Based on activated-complex theory 55 , recovery time has been defined by: where, ω is the attempt frequency (~ 10 12 s −1 ), E ads is adsorption energy, K B is Boltzmann' Constant and T is temperature. At last, based on non-equilibrium Green's function (NEGF), the IV response was examined by where, G 0 is quantum conductance and T (E, V b ) is the transmission probability of an electron incident at an energy E through the device under the bias voltage V bias , µ L and µ R are the electrostatic potentials of left and right electrodes.

Results and discussions
Adsorption of H 2 , H 2 S and HF gases on penta-PdAs 2 monolayer. In the first step, we have systematically examined the geometries of pristine penta-PdAs 2 monolayer. The optimized structure of penta-PdAs 2 is displayed in Fig. 1a. The bond length between Pd-As and As-As are 2.51 Å and 2.32 Å, respectively, which are comparable with previous reported studies 57 . To study the electronic properties such as the electronic band structure, DOS and PDOS were calculated and the obtained electronic band gap (E g ) of penta-PdAs 2 is 0.34 eV 48 .
In the second step, we have optimized the gas molecules of H 2 S, HF and H 2 on the surface of penta-PdAs 2 sheet. We have placed the gas molecules at all possible parking sites on the sheet and initially molecules are fixed at the (i) Hollow (ii) Top of Pd atom (iii) Top of As atom (iv) Bridge of Pd-As and (v) Bridge of As-As atoms. The adsorption energy of each case is reported in ESI (Table S1, ESI) and highest adsorption energy site is considered for the further calculations. Therefore, the most favorable sites of H 2 S and H 2 gases are perceived at bridge of Pd-As with adsorption distance of 2.91 Å and 2.70 Å, respectively. While HF gas is relaxed on the top of Pd atom with distance of 2.34 Å. Although, after the relaxation of that sites, the gas molecules are shifted on the top of Pd atom in all cases, which is shown in Fig. 1b-d. After the stable adsorption, the bond length between H-S, H-H, www.nature.com/scientificreports/ and H-F are 1.37, 0.77 and 0.94 Å, respectively and the bond angle of non-linear H 2 S is 92.142°, which is agreed with previous reported work 58 . Table 1 presents the adsorption energy (E ads ), adsorption distance (d) and relaxation time (τ) and type of interaction for considered H 2 S, HF and H 2 gas molecules on the surface of penta-PdAs 2 monolayer. As shown in table, the calculated adsorption energy of H 2 S, HF and H 2 gases are − 0.49, − 0.39 and − 0.16 eV, respectively. The E ads values of H 2 S, HF and H 2 were negative, suggesting that the adsorption process was all energetically favorable on the penta-PdAs 2 monolayer and all in the physisorption range. Among three gases, H 2 S (HF) has strongest with 0.49 (0.39) eV compared to H 2 , it has the weakest adsorption strength about E ads ~ 0.16 eV. Although, the obtained adsorption energy of H 2 is 18.75% higher compared to adsorbed on phosphorene (0.13 eV) nanosheet 59 . Additionally, Majidi et al. reported the HF and H 2 S detection on the twin graphene and Ti-embedded twin graphene with quoted adsorption energy of − 0.16 and − 0.22, respectively 58 . Which means, present results are quite good compared to them as H 2 S and HF gas sensor on penta-PdAs 2 sheet. On other side, the obtained adsorption energy of H 2 S gas on penta-PdAs 2 monolayer is − 0.49 eV, which is smaller than penta-BCN monolayer (− 0.797 eV) 44 , but relatively larger than functionalized graphene 58 and phosphorene (− 0.41 eV) 21 . Further, it also be noted that the recovery time (τ) plays a crucial role in sensing application, τ is also summarized in the Table 1. The obtained sequence of the recovery time (τ) for three gases are as follow: H 2 < HF < H 2 S. Although, the recovery time of H 2 S is very long owing to the strong adsorption capacity of PdAs 2 + H 2 S (0.49 eV). Hence, the low value of recovery time strongly specified that PdAs 2 + H 2 S configuration have high selectivity and can be good choice as reversible sensors of H 2 S. Moreover, the charge transfer mechanism was studied by Hirshfeld atomic population analysis and results are summarized in Table 2. The value of Q < 0 indicates that H 2 S (− 0.137 e) gas molecules accept the electrons from the PdAs 2 sheet. whereas Q > 0, in HF (+ 0.111) and H 2 (+ 0.037) indicates that both gas molecules are donating electrons to the PdAs 2 sheet as result of that holes are expected to stay in the gas molecules as depicted in Fig. 2 by the blue arrow. Figure 2a-c, shows the charge density difference (Δρ), which is obtained by the Eq. (2). From that, it can be observed that, charge is accumulated at the negative charge region (shown as green color) and charge is depleted in the positive charge region (shown as orange color) due to the reformed of the effective interface dipoles and polarization of electrons. Where adsorption energy (E ads ) is taking place to form the redistribution of electrons at the interface region. Here, among the three gases, the H 2 S has higher charge transfer between the adsorbing PdAs 2 sheet suggesting that H 2 S has effective and strong detection with high selectivity towards the penta-PdAs 2 nanosheet compared to the HF and H 2 gases.
The surface sensitivity of the 2D nanosheet could also approached by determining the differences in work function parameter (Φ) 60 as illustrate: where, V ∞ and E F are the electrostatic potential and the fermi energy level, respectively. Basically, the work function describes the minimum energy needed to dislodge an electron from the surface of penta-PdAs 2 monolayer. The calculated work function of penta-PdAs 2 with H 2 S, HF and H 2 lies in the range of 4.63 to 4.70 eV, as tabulated in Table 2 and electrostatic potential energy level is presented in in Fig. 3a-d. The red and black dashed line indicates the fermi energy level (E F ) and energy vacuum level (V ∞ ), respectively and the black double head arrow gives the work function values. The value of Φ of bare graphene is 4.5 eV which shows it as an ideal material for where work function optimization is vital 61 . As shown in Table 2, the Φ of bare penta-PdAs 2 monolayer is 4.63 eV that is slightly larger than the graphene 61 . The nature of the Φ is symmetric for H 2 S, HF and H 2 gas molecules with Φ value of 4.573 eV, 4.708 eV and 4.634 eV, respetively. Because of the adsorption of gases, the Φ is increase in all the cases compared to bare PdAs 2 monolayer demonstrating that the electrons are transferring to the vacuum level is obstructed. Therefore, the Φ can be effectively affected by the adsorption of H 2 S, HF and    Electronic properties. In order to deeper elucidate the gas sensing behavior of penta-PdAs 2 monolayer towards H 2 S, HF and H 2 gases, we analyzes the total density of states/eV (TDOS), projected density of states/eV (PDOS) and electronic band structure of gas molecules absorbed along that of bare PdAs 2 monolayer system. Figure 4a-d, shows the density of state before and after the adsorption of H 2 S, HF and H 2 gases on the PdAs 2 surface. For comparison, the electronic band struture and TDOS and PDOS also demostrated in the ESI (Figs. S1, S2, ESI). Visibly, it is confirmations no significant change either the electronic conduction or valence bands in the band structure after the adsorption of the gas molecules but the electronic band gap (E g ) is decreases from the bare counterparts. As tabulated in Table 2, the E g decreases to 0.28 and 0.30 as a result of H 2 S and HF(H 2 ) adsorption on pristine PdAs 2 monolayer, respectively. The trivial impact of H 2 S, HF and H 2 molecules in the unoccupied states of the PdAs 2 monolayer above the fermi level is highly responsible to decreases the electronic band gap due to π to π* transition at S-point after the adsorption of the gases. Additionally, the valance bands (VBs) and conduction bands (CBs) have new sub-bands are outcome from the adsorption of H 2 S, HF and H 2 gases on PdAs 2 monolayer. The presence of new sub-bands justifies the variations of the electronic energy levels due to the adsorption of three gases. Therefore, TDOS/eV and PDOS/eV in further utilized to investigate the energy levels of bare PdAs 2 prior and after adsorption of H 2 H 2 S and HF as shown in Fig. 4a-d. It can be observe from Fig. 4a-d, the significant contribution in the DOS comes from the Pd-d and As-p orbitals with a small contribution from H, S and F atoms at the fermi level. From the DOS of the H 2 adsorption (Fig. 4b), one can see that PdAs 2 monolayer after adsorbing upholds the semiconducting feature with electronic band gap of 0.30 eV and H 2 gas molecules acts as electron donor. However, there is no more influence seen at fermi level between PdAs 2 + H 2 and bare PdAs 2 monolayer. But, the 1s orbital of H atom in the H 2 is mainly hybridizied with the 4p orbital of the As atom and localized between 5 to 6 eV in the conduction bands as shown in Fig. S2a. While in case of H 2 S absorption (Fig. 4c), the highest occupied molecules orbital (HOMO) was mainly dominated by the interrelation of the H-1s and S-3p orbital of the H 2 S gas molecules and induces distinct states around 3.5 to 4.5 eV in the CBs. The same trend also follows in the adsorbing of H 2 S gas molecules on the CoOOH sheet surface 62 . The relatively higher adsoprtion energy and charge transfer of H 2 S on the PdAs 2 sheet surface was due to the hybridyzation between the Pd-4d and S-3p states near the fermi level in the VBs as depicted in Fig. S2b. Similarly, after the adsorption of the HF gas molecules (Fig. 4d), the energy around 4 to 6 eV is arised due to the mixing of the H-1s and F-2p orbital in the HOMO region. While strong contribution of the hybridization of the F-2p (− 4 to − 2 eV) orbitals and Pd-4d are responsible for the next higher adsorption energy of the PdAs 2 + HF system and acts as the electron donor near the fermi energy level as viewed in Fig. S2c. Likewise, very recently, Kaur et al. also reported the 2D janus WSSe monolayers as efficient nanosensor towards toxic HF gas molecules and the PDOS of adsorbed HF gas molecules is also contributed as same as here presented results 63 . Therefore, the remarkable ability of penta-PdAs 2 monolayer makes a potential candidate for the H 2 S gas sensing applicatons.

Electric transport properties of adsorbed H 2 S, HF and H 2 gases on penta-PdAs 2 monolayer.
To quantitatively probe the gas sensing properties of the PdAs 2 being an efficient nano sensor, non-equilibrium Green's function formalism (NEGF) was employed to analyze the current (I)-voltage (V) response 64 . The structural schematic model of sensing device set up are shown in Fig. 5. Where, the shaded region shows the elec- www.nature.com/scientificreports/ trodes which were simulated within the a 3 × 3 supercell of bare PdAs 2 monolayer. The central or scattering region exemplifies the device area, where gas adsorption take place. Figure 6a,b, exhibits the I-V characteristics of PdAs 2 before and after gas molecules exposure. Although the adsorption of H 2 S, HF and H 2 gas molecules does not affect much the electronic structure pointedly, but the charge transfer (Q) after their exposure is expected to vary due to doping of electrons or holes during the electron transportation. And these effects are revealed to be possible indication for gas sensors. As shown in Fig. 6b, In the case of bare PdAs 2 , we found that the current response in the range of micro-ampere (µA). Further, the junction remains switched off at the 0 V but seems switched on with the increasing of bias voltage. Then, noticeably above the 0 V, all the absorbed gas systems have a relevant effect on the transport from the bare one, which making all systems for gas sensing applications.
As seen in Fig. 6a, after the gas exposure the current is reducing from the bare PdAs 2 and gives response in order of nA. When bias over 0 V, the current starts to increase simultaneously up to 1.6 V for all cases. Interestingly, after the bias of 1.6 V the current response trends change slightly for all absorbed gas molecules. At V = 1.8 V, the highest current sensitivity (current of 0.32 nA) with current is observed for the PdAs 2 + H 2 S system compared to HF and H 2 system. Which could be due to acceptor nature of the H 2 S gas molecules. Further, in the H 2 adsorption (Fig. 6a), the current is significantly enhanced and reached up to 0.58 nA at the bias of 2 V. While it calibrate, for H 2 S (with current of 0.35 nA) and HF (with current of 0.33 nA) gas exposure a current reduction is about 60% and 56.89%, respectively compared to H 2 gas exposure under the bias of 2 V as summarized in the Table 3. Overall, all the results indicated that the penta-PdAs 2 monolayer could be better applicants for achieving the sensitivity and selectivity towards H 2 S compare to HF and H 2 gas molecules.

Conclusion
In summary, the noxious H 2 S, HF and H 2 gas molecules on a penta-PdAs 2 nanosheet are first time investigated within the density functional level of theory including van der Waals corrections. We systemically discovered the most favorable binding site of each molecule based on the five different adsorption positions. The adsorption energy (E ads ), charge transfer (Q), recovery time (τ) and work function (Φ) were investigated to understand performance and behavior of adsorption of the pollutant on PdAs 2 surface. The gas sensitivity order of pollutant gases was predicted as follow: H 2 S > HF > H 2 on the penta-PdAs 2 nanosheet. The obtained recovery time (τ) of the sensor to all target gases are less than 3 min. The adsorption of H 2 S and HF results in an obvious increasing in DOS near E F, which is not observed by the adsorption of H 2 . Eventually, the IV response were carried out to study the responses of bare and absorbed PdAs 2 nanosheet and the sharp rises seen at V = 1.8 V in order of H 2 S > HF > H 2 that shows the authenticated potential as efficient gas sensor. These outcome acclaim the exciting prospects of developing penta-PdAs 2 monolayer for the ultrahigh-sensitivity gas sensing nano-devices.

Data availability
The datasets generated and/or analysed during the current study are not publicly available due to privacy or other restrictions. However, it may be made available from the corresponding author on reasonable request.